Paleomagnetism from Deception Island (South Shetlands archipelago, Antarctica)

33 Pages·2016·2.25 MB·English

by  Oscar Pueyo Anchuela

1 Paleomagnetism from Deception Island (South Shetlands archipelago,
2 Antarctica), new insights into the interpretation of the volcanic evolution
3 using a geomagnetic model
4
5 B. Oliva‑Urcia1 · I. Gil‑Pena2 · A. Maestro1,2 · J. Lopez‑Martinez1 ·
6 J. Galindo‑Zaldivar3,4 · R. Soto5 · A. Gil‑Imaz6 · J. Rey7 · O. Pueyo6
7 B. Oliva‑Urcia
8 [email protected]
9 I. Gil‑Pena
10 [email protected]
11 A. Maestro
12 [email protected]
13 J. Lopez‑Martinez
14 [email protected]
15 J. Galindo‑Zaldivar
16 [email protected]
17 R. Soto
18 [email protected]
19 A. Gil‑Imaz
20 [email protected]
21 J. Rey
22 [email protected]
23 1. Departamento de Geología y Geoquímica, Facultad de
24 Ciencias, Universidad Autónoma de Madrid, 28049 Madrid,
25 Spain
26 2 Instituto Geológico y Minero de España, Ríos Rosas, 23,
27 28003 Madrid, Spain
28 3 Departamento de Geodinámica, Universidad de Granada,
29 18071 Granada, Spain
30 4 Instituto Andaluz de Ciencias de la Tierra, 18071 Granada,
31 Spain
32 5 Instituto Geológico y Minero de España, Unidad de
33 Zaragoza, Manuel Lasala, 44‑9(cid:129)‹B, 50006 Zaragoza, Spain
34 6 Área de Geodinámica Interna, Departamento de Geología,
35 Universidad de Zaragoza, 50009 Zaragoza, Spain
36 7 ESGEMAR, Espacio 4, Polígono San Luís, 29006 Málaga,
37 Spain
38 Abstract Deception Island shows the most recent exposed active volcanism in
39 the northern boundary of the Bransfield Trough. The succession of the volcanic
40 sequence in the island is broadly divided into pre- and post-caldera collapse
41 units although a well-constrained chronological identification of the well-defined
42 successive volcanic episodes is still needed. A new paleomagnetic investigation
43 was carried out on 157 samples grouped in 20 sites from the volcanic deposits
44 of Deception Island (South Shetlands archipelago, Antarctic Peninsula region)
45 distributed in: (1) volcanic breccia (3 sites) and lavas (2 sites) prior to the
46 caldera collapse; (2) lavas emplaced after the caldera collapse (10 sites); and
47 (3) dikes cutting pre- and the lowermost post-caldera collapse units (5 sites).
48 The information revealed by paleomagnetism provides new data about the
49 evolution of the multi-episodic volcanic edifice of this Quaternary volcano,
50 suggesting that the present-day position of the volcanic materials is close to
51 their original emplacement position. The new data have been combined with
52 previous paleomagnetic results in order to tentatively propose an age when
53 comparing the paleomagnetic data with a global geomagnetic model. Despite
54 the uncertainties in the use of averaged paleomagnetic data per volcanic units,
55 the new data in combination with tephra occurrences noted elsewhere in the
56 region suggest that the pre-caldera units (F1 and F2) erupted before 12,000
57 year BC, the caldera collapse took place at about 8300 year BC, and post-
58 caldera units S1 and S2 are younger than 2000 year BC.
59
60 Introduction
61
62 he most common methods used for dating volcanic rocks are radiometric,
63 especially potassium–argon (K/Ar), argon–argon (40Ar/39Ar), uranium–lead
64 (Pb/U) and radiocarbon dating (14C). Nevertheless, in many situations
65 radiometric dating is not possible, or difficult: in case of evident post-
66 depositional alteration, or when the minerals content is not suitable (e.g., low K-
67 content), or in case of rocks too young to show isotopic decay. Both 40Ar/39Ar
68 and K/Ar methods can lose accuracy in young (Holocene) and less evolved
69 products, especially in K-poor volcanic rocks (i.e., basalts erupted in most intra-
70 oceanic volcanoes). 14C is a powerful dating tool for dating last millennia, and
71 indeed, it is the most suitable and commonly used for Holocene sediments, but
72 organic material or soils are required, which can be absent in many cases in
73 Antarctic environments.
74
75 An additional complication for 14C dating of volcanic events in the
76 Antarctic is the uncertainty over radiocarbon reservoir effects in that region
77 which yielded older than expected ages in lake and marine sediments (Björck et
78 al. 1991a, b; Ingólfsson et al. 1992; Hjort et al. 1997; Takahashi et al. 1999; Hall
79 and Henderson 2001). Moreover, when dealing with volcanic environments, in
80 some cases soils or lahars can also bear organic materials, which form at
81 favorable climatic conditions and when the eruption rate is low enough for a soil
82 to develop. Therefore, in the case of active volcanoes with high frequency of
83 eruptions, soils cannot form and the 14C method cannot be used. When
84 radiometric dating is not possible, variations in the geomagnetic field recorded
85 in volcanic rocks can be used as a dating tool for eruptions. The advantage of
86 the paleomagnetic dating method is that it can be used on wholerock samples
87 and does not need particular petrographic/ chemical characteristics (magnetic
88 remanence is generally carried by titanomagnetite series of minerals, ubiquitous
89 in volcanic rocks). The reliability of paleomagnetic dating depends on the
90 availability of geomagnetic models and on the accuracy of paleomagnetic
91 determinations in the región under investigation.
92
93 Deception Island, South Shetland Islands, off the northern Antarctic
94 Peninsula is a Quaternary volcano with an inner bay (Port Foster), which shows
95 the most recent activity in the Bransfield Strait region (e.g., Baker et al. 1975).
96 The products of numerous eruptions have been recorded as a result of detailed
97 mapping (e.g., Smellie 2002). Although their relative ages are well defined, their
98 absolute chronological ages are not and only the ages of a few historical
99 eruptions are known (Smellie 2002). The remainder of the island lacks a
100 chronology and, in particular, the age of the major caldera-forming eruption,
101 which is the most important eruptive episode to have occurred, is unknown
102 (Smellie et al. 1984; Smellie 2002; Martí et al. 2013). The island is a shield
103 volcano, with a submerged basal diameter of about 30 km and a summit at
104 Mount Pond (540 m), which rises ~1.5 km from the sea floor (Smellie 1990).
105 The current morphology of the island shows a central flooded caldera and a
106 variety of craters, lavas and different volcanic deposits (e.g., tephra, tuffs,
107 scoria) partially covered by ice (López-Martínez et al. 2000). Classically,
108 authors who have studied the volcanic sequences in Deception Island
109 determined a caldera collapse, which allows separating the volcanic deposits in
110 pre- (or pre- and syn-) and post-caldera collapse (e.g., Hawkes 1961; Baker et
111 al. 1975; Birkenmajer 1992; Smellie and López-Martínez 2000; Martí et
112 al.2013). The pre-caldera deposits include the Port Foster Group (F1, F2 and
113 F3 formations in this work), and the post-caldera deposits include the Mount
114 Pond Group (S1 and S2) (following Smellie and López-Martínez 2000). The
115 collapse was considered to be caused by concentric faults (Hawkes 1961;
116 Baker et al. 1975; Birkenmajer 1992) although more recent geophysical studies
117 have shown that the gravimetric and magnetic anomalies are not related to a
118 typical circular structure of a classic volcanic caldera but rather reveal a NE–SW
119 linear trend in the crustal mass (Ortiz et al. 1992; Funaki et al. 2012; Martí et al.
120 2013).
121 A large eruption is likely related to the caldera collapse, although the
122 collapse would have exploited any preexisting tectonic features (Smellie 2002).
123 The main objective of this paper is to constrain relative ages of the different
124 igneous rock units after comparing the averaged paleomagnetic data per unit to
125 global geomagnetic models (Pavón-Carrasco et al. 2014) once we decipher
126 whether the present-day positioning of the volcanic deposits of Deception Island
127 (lavas, pillow lavas with scoria, and pyroclastic lavas) is original or other post-
128 cooling processes [tectonic deformation or tilting due to the evolution of the
129 volcanic edifice after the thermoremanent magnetization (TRM) acquisition]
130 have modified the attitude of the volcanic deposits to their present position
131 using paleomagnetic data. This study has been carried out by combining 20
132 new sampling sites from Deception Island rocks with previous data from
133 Baraldo et al. (2003) (17 sites).
134
135 Geological setting, studied material and previous paleomagnetic data
136
137 Deception Island is located in the boundary between the South Shetland
138 crustal block and the southwestern part of the Bransfield Trough, a young (<1.4
139 Myr), narrow, elongated, ENE–WSW basin located between the South
140 Shetlands archipelago and the Antarctic Peninsula (Fig. 1a, b; Barker et al.
141 1991). The Bransfield Trough is a marginal basin situated at the rear of the
142 inactive South Shetland Islands volcanic arc (Fretzdorff et al. 2004), formed as
143 a consequence of the subduction of the former Phoenix Plate under the
144 Antarctic Plate during Mesozoic–Cenozoic times (Dalziel 1984) and the
145 interaction with the westward motion of the Scotia–Antarctic plate boundary
146 along the South Scotia Ridge (González-Casado et al. 2000). The Phoenix
147 Ridge spreading ceased ~4 Myr ago (Lawver et al. 1996), although subduction
148 continued along the South Shetland Trench (Maldonado et al. 1994). The
149 Bransfield Trough extends toward the NE with the western part of the E–W
150 trending South Scotia Ridge through a deep trough interpreted as a
151 transtensional basin (Canals et al. 1992; Acosta et al. 1994; Galindo-Zaldívar et
152 al. 1996, 2004). The East Scotia Sea is an active back-arc basin. Deception and
153 Bridgeman islands are highs that divide the Bransfield Trough into western,
154 central and eastern basins. The leftlateral plate boundary (along the South
155 Scotia Ridge) forms the southern boundary of the Scotia Plate (Fig. 1c).
156
157 Several volcanic centers in the northern Antarctic Peninsula have the
158 potential to disperse volcanic ash over large areas. The record of Quaternary
159 volcanism from Bransfield Trough, the South Shetland Islands and surrounding
160 seas, mostly in the form of tephra deposits, identifies Deception, Penguin and
161 Bridgeman islands as the most active sources with lesser contributions from
162 James Ross Island, seamounts and ridges, some of which may also be active
163 (Kraus et al. 2013; Weaver et al. 1979; González-Ferrán 1985; Fisk 1990; Keller
164 and Fisk 1992; Gràcia et al. 1996; Smellie et al. 2008).
165
166 The volcanic ridges and spurs in the Bransfield Trough are parallel to the
167 NE–SW basin axis, which is divided by strike-slip faults (oriented mainly NW–
168 SE) (Fig. 1c; Grad et al. 1992). An extensional fault system orientated NW– SE,
169 together with a conjugate group of NE–SW normal oblique-slip faults, is present
170 in the northern margin of the central Bransfield Basin (González-Casado et al.
171 1999). The recent tectonic evolution of the Bransfield Trough is controversial
172 and explained by three different models: (1) Opening of the basin may be
173 related to passive subduction of the former Phoenix Plate and rollback of the
174 South Shetland Trench (e.g., Smellie et al. 1984; Maldonado et al. 1994;
175 Lawver et al. 1995); (2) the sinistral movement between the Antarctic and
176 Scotia plates may be causing an oblique extension along the Antarctic
177 Peninsula continental margin. This extension generates the Bransfield Trough
178 and defines the South Shetland tectonic block (Rey et al. 1995; Klepeis and
179 Lawver 1996; Lawver et al. 1996; González- Casado et al. 2000); or (iii) both
180 mechanisms cited before could occur simultaneously (Galindo-Zaldívar et al.
181 2004; Maestro et al. 2007).
182
183 Volcanism in Deception Island evolved during the Quaternary from pillow
184 lavas to strombolian and phreatomagmatic subaerial eruptions. The volcanic
185 deposits are divided into pre-caldera and post-caldera units (Smellie and López-
186 Martínez 2000; Smellie 2001, 2002; Fig. 2). The pre-caldera units are named
187 Port Foster Group, which includes from bottom to top Fumarole Bay (F1),
188 Basaltic Shield (F2) and Outer Coast Tuff (F3) formations. They include
189 subaqueous strombolian (likely fire-fountaining) deposits (F1), subaerial lavas
190 (F2) and pyroclastic density current deposits (F3) that record the progressive
191 evolution from submarine to subaerial volcanism (Smellie and López-Martínez
192 2000) and include mainly mafic rocks (Galé et al. 2014; and references therein).
193 The Fumarole Bay Fm is interpreted as mostly subaqueous and originated from
194 several coalesced centers (Hawkes 1961; Smellie 2002). The Basaltic Shield
195 Fm is basically composed of dry effusive and minor pyroclastic subaerial
196 material. It is interpreted as a small shield volcano with at least two centers
197 (Smellie 2002). The Outer Coast Tuff Fm is the result of a large eruption just
198 prior to the caldera collapse and probably genetically related to the caldera
199 collapse (Smellie 2002). This unit consists mainly of pyroclastic material, and it
200 is widespread distributed through the island, draping unconformably on the
201 Fumarole Bay and Basaltic Shield Fms. It is cut by a caldera fault, indicating
202 that it represents products of the final eruption prior to (or possibly during)
203 formation of the caldera (Smellie 2001, 2002). It was also suggested that the
204 vent(s) of the Outer Coast Tuff Fm were below sea level (Smellie 2002). These
205 deposits scarcely crop out since post-caldera materials cover them but they are
206 well exposed in (largely inaccessible) cliffs of the outer coast and in some inner
207 caldera wall sections. Martí et al. (2013) consider Outer Coast Tuff Fm as a
208 syn-caldera collapse deposit, suggesting that the caldera subsidence occurs
209 prior to its deposition. The post-caldera units consist of small-volume
210 phreatomagmatic eruptions scattered across the island (hydrovolcanic tephra,
211 lavas, tuff cone and maar deposits) and magmatic eruptions (strombolian scoria
212 and lavas) of basaltic to dacitic composition (Smellie 2002; Galé et al. 2014 and
213 references therein), mainly originated from multiple centers and from bottom to
214 top include Baily Head and Pendulum Cove formations (Smellie and López-
215 Martínez 2000; Smellie 2001). Almost time equivalent to the previous
216 formations are Kendall Terrace Member (at least partly older than Baily Head
217 Fm) and Stonethrow Ridge Fm (Mt Kirkwood Member), which is time equivalent
218 to Pendulum Cove Fm.
219
220 Age of Deception Island
221
222 The relative ages of the individual volcanic units of Deception Island are
223 comparatively well known; however, their absolute ages are not. Previous
224 paleomagnetic data indicate that all rocks cropping out record normal magnetic
225 polarity, what leads to infer an age younger than 780 ky (time of the last
226 inversion of the magnetic poles; Valencio et al. 1979; Baraldo et al. 2003).
227 Attempts to date the volcanic events of Deception Island include
228 radiometric dating of rocks from the island and tephras from ice, in addition to
229 lacustrine and marine sediments around the Antarctic Peninsula. Radiometric
230 K–Ar dating on whole rock suggests an age younger than 150 ky (±50 ky) for
231 the subaerial volcanism of gray lava from Telefon Ridge (Keller et al. 1991),
232 although it is considered too old and likely unreliable by Smellie (2001),
233 because the sampled area corresponds to post-caldera eruptions the
234 Stonethrow Ridge Fm, which includes historically documented eruptions from
235 the XIX and XX centuries. Other information related to absolute dating comes
236 from 14C of organic material associated with tephras from the Antarctic
237 Peninsula area (ice, lakes and marine sediments). The geochemical
238 compositions of the glass shards show that Deception Island is the major
239 source of Quaternary tephra horizons in the northern Antarctic Peninsula region
240 (Smellie 1990, 1999; Björck et al. 1991a; Hodgson et al. 1998; Pallàs et al.
241 2001; Lee et al. 2007). Moreton (1999) analyzed a megascopic ash layer
242 recovered from six sediment cores from the Scotia and Weddell seas. This
243 deposit represents the most extensive late Quaternary tephra known to be
244 deposited in the northern Antarctic Peninsula region and Deception Island is
245 unambiguously identified as the source. The megascopic tephra layer has been
246 14C dated as 10,670 BP (Moreton 1999), and it is tentatively attributed to the
247 eruption that immediately preceded and directly led to the caldera formation on
248 Deception Island. The dating of the megascopic tephra layer is tentative as
249 there are large discrepancies between the radiocarbon ages obtained from the
250 sediments above and below the tephra layer in different cores and there are still
251 large uncertainties regarding the magnitude of the marine reservoir effect at the
252 time of the eruption (Moreton 2015 pers. comm.). Below the megascopic tephra,
253 five other tephras can be correlated with varying extents between the central
254 Scotia Sea cores. Deception Island has been identified as the source of all
255 correlatable tephras. Three of these tephras have been radiocarbon-dated to
256 21,660 BP, 26,400 BP and 35,400 BP (the two oldest correlatable tephras have
257 not been directly dated; Moreton 1999). Smellie (2001) considers that exposed
258 rocks in Deception Island appear to be Late Pleistocene-Recent (probably <100
259 ka).
260
261 Studied materials and previous paleomagnetic data
262
263 The new paleomagnetic sites of the pre-caldera units (Smellie and
264 López-Martínez 2000) are located in volcanic breccia and pillow lava of the
265 Fumarole Bay Fm (F1, 4 sites) and in lavas of the Basaltic Shield Fm (F2, 1
266 site) (Tables 1, 2, 3; Figs. 2, 3). These outcrops are located in the no-collapsed
267 area of the volcano, and the sampled lavas show low to moderate dips. The
268 new paleomagnetic sites of the post-caldera units (Smellie and López-Martínez
269 2000) are located in lavas and, from younger to older, in the Kendall Terrace
270 Member (scoria and lavas) (S1), in the lavas of the Collins Point cones (P2,
271 which are partly equivalent in age to S2), and in the Stonethrow Ridge Fm,
272 consisting of strombolian scoria and lavas (S2). The S1 unit drapes the caldera
273 rim and has cascaded down the inner surface (Smellie 2002). It has been
274 studied in 6 sites all around the island. The dikes were probably emplaced
275 contemporaneously to the S1 unit, and in the sampled sites cut pre-caldera
276 units [Fumarole Bay Fm and Outer Coast Tuff Fm, F1 and F3 units,
277 respectively, of Smellie and López-Martínez (2000), Smellie (2002)] and post-
278 caldera units [Baily Head Fm, P1 unit of Smellie and López-Martínez (2000),
279 Smellie (2002)]. The P2 unit was sampled in three sites in Collins Point, to
280 investigate possible local post-cooling deformations in the lavas as it was
281 suggested by the outcrop appearance. The P2 unit is partly equivalent in age to
282 S2 unit, but this last formation extended longer in time; however, for the
283 discussion of the results, both units are combined into S2 unit. Later historical
284 volcanic episodes are registered from 1829 to 1970 AD (from P3 to P8 units but
285 considered S2 for this study) following descriptions of Smellie and López-
286 Martínez (2000). Seventeen sites studied by Baraldo et al. (2003) were also
287 taken into consideration (Table 4). These sites come from F2 (2 sites), F3 (6
288 sites), S2 (8 sites) units of Smellie and López-Martínez (2000), and one site
289 from 1840 AD (considered together with S2 sites). Previous paleomagnetic
290 studies performed in the Bransfield Trough, South Shetland Islands, Antarctic
291 Peninsula and Patagonia pursued two different goals. On the one hand, there
292 are the paleogeographical reconstructions that seek to unravel the tectonic
293 evolution of the area since the Paleozoic (e.g., Poblete et al. 2011). And on the
294 other hand, the paleomagnetic studies that focus in the more recent volcanic
295 rocks in order to study the evolution of the Earth’s magnetic field and its secular
296 variations (Baraldo et al. 2003). More specifically, previous paleomagnetic
297 studies in Deception Island conclude that the low coercive magnetic minerals in
298 the lavas averaged out the paleosecular variations, being the calculated mean
299 direction no more than 2° far from the geocentric axial dipole direction model
300 (Baraldo et al. 2003).
301
302 Methods and paleomagnetic analysis
303
304 The sampled sites were chosen looking for the most complete
305 stratigraphic record of the Deception Island volcanism. Mostly, lavas and dikes
306 have been studied, in addition to 4 sites from F1 unit (volcanic breccias—3
307 sites—and pillow lavas—1 site—). In the lavas, sampling was performed far
308 from their external areas to avoid problems linked to a faster cooling when the
309 lava still moves and/or possible interactions with underneath and subsequent
310 volcanic episodes, considering that in certain cases the underlying magnetized
311 bodies can affect the paleomagnetic records, for example, during periods of
312 reduced field intensity as in reversals (Valet and Soler 1999). In two sites (5,
313 12), this has not been possible and we sampled the scoriaceous crust of the
314 lava. The sampling was done in the field with a portable gasoline drill machine
315 cooled with water. Drilled cores were oriented with an inclinometer and a
316 compass. The 157 standard samples (2.5 cm in diameter and 2.1 cm in height),
317 from 20 sites (Fig. 2), were thermally demagnetized at the paleomagnetic
318 laboratory of University of Barcelona- CSIC using Schonsted and MMT80
319 furnaces. Furnaces were used to progressively heat the samples (from an initial
320 room temperature to 580 °C) in 11–14 steps (every 50–20 °C) until complete
321 demagnetization has occurred. The remanence was measured in a JR5
322 (AGICO Inc. Brno, Czech Republic). A stable component is calculated with
323 good quality in 92 % of the samples. Stable directions are defined by visual
324 inspection of the demagnetization diagrams with Remasoft 3.0 (Chadima and
325 Hrouda 2006) and VPD (Ramón and Pueyo 2014). The stable component is
326 computed at high temperature with more than 6 steps and maximum angular
327 deviations (MADs) less than 5°. The method of calculation is by principal
328 component analyses (PCA; Kirschvink 1980). The maximum unblocking
329 temperatures range from 350 to 580 °C, which suggests titanium-rich magnetite
330 and magnetite as carriers of the magnetization (Baraldo et al. 2003; Pueyo-
331 Anchuela et al. 2014). Stereoplots were done with Cardozo and Allmendinger
332 (2013) OSXSteronet software. Acquisition of isothermal remanent
333 magnetization curves (IRM) were performed on selected samples with an
334 Impulse Magnetizer IM10-30 (ASC Scientific) applying a maximum field of 1 T
335 to obtain more information about the ferromagnetic carriers.
336
337 Paleomagnetic results
338
339 The paleomagnetic characteristic component has been calculated
340 successfully in 88 % of the analyzed samples. The ferromagnetic carrier is of
341 very low coercivity as seen in the acquisition curves of the isothermal remanent
342 magnetization (Fig. 4) and by previous results (Baraldo et al. 2003). The
343 average of the natural remanent magnetization (NRM) at every site ranges from
344 0.05 to 20 A/m, except for site 12, which is almost 40 A/m. The NRM average
345 has a value of 9.1 with a standard deviation of 4.4 A/m (without taking into
346 account site 12; Fig. 5).
347
348 Demagnetization diagrams of selected samples are shown in Fig. 6, and
349 the average component for every site is in Table 3. The mean inclinations vary
350 from −56° (site 5) to −85° (site 9), and declinations from 319° (site 1) to 16° (site
351 5). The expected inclination for Deception Island at latitude 63°S is: INC = 75.7°
352 following the dipole formula for the Earth magnetic field (tan λ = 0.5 * tan INC,
353 where λ is the latitude). Paleomagnetic data at high latitudes have the drawback
354 of the error associated with high inclinations since the paleomagnetic inclination
355 is related to latitude. The error in declination for a site (averaging 8 or 7
356 samples) is calculated dividing the α95 by the cosine of the magnetic inclination
357 (εDEC ~ α95/ cos INC). Therefore, the higher the magnetic inclination, the
358 higher the uncertainty of the declination. The fisherian parameter α95
359 represented by the semi-apical angle of a cone that revolves around the
360 average and where the 95 % of the data (with normal distribution) is contained,
361 is also a measure of the concentration of the average for a site; therefore, the
362 higher the α95, the higher the error in declination. Errors in the α95 for
363 Deception Island are indicated in Table 3. In average, they are very low, α95 <
364 5°, except for the oldest rocks (F1 unit), which are volcanic breccias (and one
365 site in pillow lavas). Consequently, the error in paleomagnetic declination for the
366 latitude of Deception Island will be (in average) equal to 5° divided by cosine of
367 75.7° (cosine of the expected inclination), which is ~19°.
368
369 Discussion
370
371 The paleomagnetic data, apart from detecting rotations (or lack of them)
372 in multiple tectonic scenarios (e.g., Van der Voo 1993; Sussman and Weil
373 2004), can have another application in volcanic contexts, i.e., use the variation
374 of the geomagnetic field recorded in volcanic rocks as a correlating tool of
375 eruptions, so long as the volcanic units have not undergone tilting through post-
376 depositional volcanic or unrelated tectonic processes (Doell and Cox 1963). In
377 addition, paleomagnetic data from well-dated volcanic (e.g., Carlut et al. 2000;
378 Tauxe et al. 2004) and from archaeomagnetic materials (e.g., Gómez-Paccard
379 et al. 2006) are used to construct geomagnetic models (e.g., Pavón-Carrasco et
380 al. 2014) that allow changes of the Earth’s magnetic field at the surface to be
381 calculated. Therefore, any undated paleomagnetic data can be compared at
382 any location with the geomagnetic model for dating purposes (e.g., Gómez-
383 Paccard and Beamud 2008; Speranza et al. 2008; Kissel et al. 2015).
384
385 The time of the acquisition of the thermoremanent magnetization (TRM)
386 (paleomagnetic vector) in a volcanic deposit occurs when the material (lavas,
387 ignimbrites, pyroclasts, tephras) cools down below TRM acquisition
388 temperature, which is the Curie temperature. Just below the Curie temperature,
389 the ferromagnetic s.l. minerals (Fe–Ti-oxides) change from paramagnetic to
390 ferromagnetic s.l., precisely at that time they record the Earth magnetic field.
391 The Curie temperature depends more specifically on the chemical composition
392 of the ferromagnetic mineral; e.g., for pure magnetite, it is 580 °C and
393 decreases as titanium enters in the structure (Dunlop and Özdemir 1997).
394 Therefore, the acquisition time of the remanent magnetization is close to the
395 eruption time, since eruption temperatures range from 1600 °C (more basic
396 lavas) to 900 °C (Griffiths 2000 and references therein). As an example, in
397 Hawaii a lava flow with an eruption temperature of about 1150 °C in 52 min
398 descends to ~400 °C (Flynn and Mouginois-Mark 1992).
399
400 The cooling rate for dikes is higher, but also depends on diffusivity, depth
401 of emplacement, composition and especially dike thickness. Depending on the
402 last condition, time of cooling obtained from a thermal modeling in aplite–
403 pegmatite dikes can vary from ~9 years to 5 days for a 25-mto 1-m-thick dike,
404 respectively (Webber et al. 1999). For ignimbrites and other volcanic breccias,
405 the acquisition of a stable characteristic remanent magnetization also depends
406 on the time of cooling down below the temperature for the TRM acquisition.
407 Therefore, paleomagnetic data are used to calculate temperatures of
408 emplacement (Porreca et al. 2006). Recently, it has been suggested that
409 densely welded pyroclastic deposits record accurately the ambient geomagnetic
410 field direction at the time of emplacement; on the contrary, non-welded
411 pyroclastic deposits show large confidence limits or deviations from their
412 expected directions due to modification of remanence direction introduced from
413 random rotations of remanence-carrying material during syn- or post-
414 depositional stages (Uno et al. 2014).
415
416 Secular variations, age and cooling rate differences among sites
417
418 When magnetic vectors of the same unit (and therefore of the same age)
419 group well in the in situ position, the displacements of the volcanic deposits
420 after cooling down below TRM acquisition temperature are minimal. On the
421 contrary, when a local post-emplacement rotation occurs, the paleomagnetic
422 vectors will be more scattered in the in situ position than in the previous case.
423 Conversely, due to geomagnetic secular variation, when two distinct volcanic
424 deposits show statistically separate and well-characterized TRMs, and
425 deformation has not taken place, that indicates a significant time interval (>100
426 years) elapsed between deposits, confirming the existence of two different
427 volcanic events (Paquereau-Lebti et al. 2008). Baraldo et al. (2003) have
428 already determined that PSV are averaged out in Deception Island. In
429 Deception Island, we compare only sites within the same volcanic unit in a
430 broad sense, with respect to time. Since there is a lack of a good age constraint
431 in rocks cropping out at Deception Island, a total of five main groups are
432 considered, following divisions from Smellie and López- Martínez (2000): (1)
433 three pre-caldera groups or units, F1, F2 and F3, and (2) two post-caldera
434 groups or units, S1 (including dikes) and S2. For the pre-caldera studied
435 materials, F1 was subaqueously originated from different vents, some of which
436 were co-eruptive (Smellie 2001). The F1 unit is divided into three members that
437 are, from older to younger, F1a (hyaloclastite breccia and lava), F1b
438 (palagonitized scoria member: lapillistone and coarse grain lapilli tuff) and F1c
439 (well-stratified yellow lapillistone and lapilli tuff). They can be recognized in the
440 field as three distinct deposits that formed progressively within single vents as a
441 result of progressive shoaling and variations in the proportion of external water
442 available (Smellie 2001, 2002). The lavas sampled in the F2 unit (a subaerial
443 lava shield) seems originated in one single vent, but that is not so clear for F3,
444 where the volume of erupted material is calculated as 30–90 km3, quite large
445 for the island and with significant involvement of water (Baraldo and Rinaldi
446 2000; Smellie 2001, 2002; Martí et al. 2013). Therefore, any large differences in
447 magnetic declination or inclination among sites of these units might be related
448 to tilting. The post-caldera collapse units encompass a higher variability in
449 eruption timing (especially S2 group) and also in space; they originated from
450 multiple centers (Smellie and López-Martínez 2000); consequently, any
451 differences in the magnetic signal (declination and inclination) among sites can
452 be attributable not only to possible tilting but also to secular variations due to
453 age difference. On top of that, possible discrepancies among sites or within
454 sites (higher scattering of paleomagnetic data within a site) can be due to very
455 fast cooling before final deposition, then the TRM will be scattered or TRM can
456 show at least 2 components, and the characteristic component can be scattered
457 above certain temperature (Porreca et al. 2006), this will be more probable to
458 occur in volcanic breccia or scoriaceous lavas than in lavas, since cooling rates
459 are expected to be faster in the former.
460
461 Paleomagnetic data are “untilted”
462
463 The five volcanic units are represented in Fig. 7 and Table 5. The
464 magnetic inclination in Deception Island is large enough to likely mask subtle
465 and local tilting among volcanic units [as seen in Gil-Imaz et al. (2010) and
466 Pueyo- Anchuela et al. (2014)]. Large tilting variations affecting declination or
467 inclination values have been detected in some site mean values in Baraldo et
468 al. (2003). Rotation errors around a vertical axis will be larger in places with
469 high magnetic inclination (as in Deception Island) because the error in
470 paleomagnetic declination values for an average site depends on the cosine of
471 the inclination. However, average values per unit from the new sites show good
472 clustering: α95 less than 6 (except for F1 sites) and k (precisión parameter)
473 higher than 198 (except for S2 and F1). α95 is the angle indicating that the
474 unknown true mean direction lies within α95 of the calculated mean, and k value
475 is higher and higher as the distribution is more and more concentrated at a point
476 (Butler 1992). Therefore, paleomagnetic averaged values for F2, F3 and S1
477 suggest that sites among those units are not largely tilted. The large scattering
478 of F1 sites can be explained considering that the lithology is indurated volcanic
479 breccia (all sites but one, which is taken in pillow lavas). The aerial volcanic
480 breccia cools down faster than lava flows and might begin to acquire the TRM
481 before the deposition of the unit ceases (Porreca et al. 2006). Our interpretation
482 for F1 unit differs from the aerial deposits, since aerial welded tuff deposits
483 provide clustered paleomagnetic data (Uno et al. 2014). It is difficult to detect
484 any possible tilting with F1 unit, but it is considered unlikely since F2 and F3
485 (also pre-caldera collapse units) show good clustering; then no large tilting in
486 the pre-caldera collapse units is occurring. Baraldo et al. (2003) found two sites
487 within F2 unit with anomalous inclination values, suggesting some local tilting.
488
489 The possibility of recording different secular variations among sites with
490 the F1 unit could be also another reason for the scattering observed among the
491 4 sites. However, the samples are from a single upwardly evolving eruptive
492 structure (i.e., different stages from the same eruptive episode), and then the
493 scattering is most probably related to the fast cooling in contact with water
494 during ejection (Smellie 2001, 2002). The F3 unit is also a subaqueous deposit